Thermal ink-jet printers operate by employing a plurality of resistor elements to expel droplets of ink through an associated plurality of nozzles. In particular, each resistor element, which is typically a pad of resistive material measuring about 50 .mu.m.times.50 .mu.m, is located in a chamber filled with ink supplied from an ink reservoir. A nozzle plate, comprising a plurality of nozzles, or openings, with each nozzle associated with a resistor element, defines part of the chamber. Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, fabric, or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. By proper selection of the signals, alphanumeric and other characters are formed on the print medium.
The tight tolerances of the nozzles (typically 50 .mu.m diameter) require that the ink not clog the nozzles. Further, repeated firings of the resistor elements, which must withstand about 10 million firings over the life of the ink cartridge, can result in fouling of the resistor element. Finally, the ink composition must be capable of interacting with the print medium, especially paper, to penetrate the paper without undue spreading, and they should be smear resistance and waterfast.
Inks are known which possess one or more of the foregoing properties. However, few ink compositions are known that possess all the foregoing properties, since an improvement in one property often results in the degradation of another property. Thus, many inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate response in each of the foregoing considerations.
Further, with regard to preparing waterfast inks, prior work has concentrated on altering the dye molecule itself, such as modifying the backbone thereof or adding various functional groups thereto. While such efforts result in inks having improved waterfastness, nonetheless, organic reactions must be performed, which drives up the cost of the dye, and hence the ink.
European Patent Application 88301275.9 discusses the use of cyclodextrins in a recording liquid to enhance print quality. The inks comprise water-soluble dyes, with a maximum concentration of about 80 wt. % water.
It is desired to prepare an ink having a level of waterfastness achievable only by use of water-insoluble dyes and using commercially-available dyes and suitable additives. At the same time, it is important to develop ink formulations which have the desired improved waterfastness, but not at the expense of other properties.